Optical filter, manufacturing method thereof, and planar lightwave circuit using the same

ABSTRACT

An optical filter includes a body of a polyhedron structure in which a recess having a predetermined depth from one side surface is exposed on a front surface and a rear surface and a multi-layer thin film which is deposited on the front surface of the body to cover an exposed portion of the recess of the body.

CLAIM OF PRIORITY

This application claims priority to an application entitled “OpticalFilter, Manufacturing Method Thereof, And Planar Lightwave Circuit UsingThe Same,” filed in the Korean Industrial Property Office on Oct. 29,2004 and assigned Serial No. 2004-87319, the contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical filter, particularly to anoptical filter which can be mounted in a planar lightwave circuit, etc.

2. Description of the Related Art

An optical filter can be manufactured by various methods depending onthe objects and functions of the filter. Some functions includenon-reflecting, high reflecting, bandwidth transmitting, or wavedivision multiplexing or demultiplexing.

Advancement in data communications and digital media has also led to theadvancement in optical devices. An example can be seen in thedevelopment and use of planar lightwave circuits that has led tominiaturization and high integration of optical devices. The deviceperforms many functions by integrating number of different types ofoptical filters.

FIG. 1 is a plain view of a planar lightwave circuit in which aconventional optical filter is mounted. Referring to FIG. 1, the planarlightwave circuit 100 includes first and second waveguides 111 and 112,a recess 120 positioned between the first and second waveguides 111 and112, an optical filter 130 positioned in the recess 120. First andsecond waveguides 111 and 112, which provide paths for lights, arecomprised of an upper clad, an active layer, and a lower clad stackedsequentially on top of a semiconductor substrate (not shown).

The recess 120 is formed by dicing. The first and second waveguides areseparated by a predetermined distance due to the recess 120 thattraverses through lower clad, the active layer, and the upper clad. Thedistance between the first and second waveguides is determined by thethickness of the optical filter 130 that is to be inserted.

A light proceeding through the first and second waveguides 111 and 112is diversed in the recess at a predetermined angle. As the distancebetween the first and second waveguides 111 and 112 gets larger, more ofthe light gets diversed and more of the light gets lost. Particularly,if the separation caused by the recess in a conventional single modewaveguide exceeds 30 μm, the engagement loss is substantial as most ofthe light signal is diversed. Therefore, a general optical filterinserted in the recess 120 should have a thickness of about 15 to 30 μm,and an optical thin film filter which uses a polymide substrate whichcan have a thickness of under or over 10 μm.

The optical filter 130 can be formed on a substrate of a polymermaterial by depositing a plurality of dielectric mediums. The opticalfilter 130 is settled down in a recess 120 and adhered to the recess 120by a thermosetting medium or ultraviolet rays of high polymer materialcapable of matching the index of refraction between the first and secondwaveguides 111 and 112. Epoxy-based or silicon-based material is usedfor the high polymer. The recess 120 is formed by sawing.

The optical thin film filter described heretofore, however, has severalproblems. First, it becomes more difficult to treat the film as thefilm's thickness decreases, and such difficulty leads to rise inmanufacturing cost. Second, low thickness of the film exposes it tophysical damages under a slight external force. Such exposure hindersthe films from being positioned on the planar lightwave circuit to beprocessed in an optical axis arranging process.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above-mentionedproblems occurring in the prior art and provides additional advantages,by providing an optical filter that can be mounted in a planar lightwavecircuit and that can be produced by an automated process.

In order to accomplish this object, an optical filter is provided. Theoptical filter includes a body of a polyhedron structure with a recesswith predetermined depth from one side surface of the body havingexposed front surface and rear surface and a multi-layer thin filmformed on the front surface of the body to cover the exposed portion ofthe recess of the body.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features and advantages of the present invention will beapparent from the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a plain view showing a planar lightwave circuit in which aconventional optical filter is mounted;

FIG. 2 is a view showing an optical filter according to a firstpreferred embodiment of the present invention;

FIGS. 3 a and 3 b are views explaining the optical filter shown in FIG.2 in manufacturing steps;

FIG. 4 is a view showing an optical filter according to the secondpreferred embodiment of the present invention;

FIG. 5 is a view explaining manufacturing processes of the opticalfilter shown in FIG. 4; and

FIGS. 6 to 8 are views showing a state in which an optical filteraccording to the second preferred embodiment of the present invention isengaged with a planar lightwave circuit.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. For the purposes of clarity andsimplicity, a detailed description of known functions and configurationsincorporated herein will be omitted as it may make the subject matter ofthe present invention unclear.

FIG. 2 is a plain view of an optical filter according to the firstembodiment of the present invention. Referring to FIG. 2, the opticalfilter 200 includes a body 210, a recess 211, a multi-layer thin film230 and oxidation films 220 a and 220 b. The body is of a polyhedronstructure containing the recess 211 with a predetermined depth from oneside surface of the body 210. The front and rear surfaces of the body210 and the recess 211 are exposed. A multi-layer thin film 230, alongwith one oxidation film 220 a, is deposited on the front surface of thebody 210 to cover the exposed front surface of the recess 211 of thebody 210. One other oxidation film 220 b covers the exposed rear surfaceof the body 210.

The body 210 can be manufactured with a wafer made from silicon basedmaterial. The oxidation films 220 a and 220 b are formed on bothsurfaces of the body by using high-temperature processing. If theoxidation films 220 a and 220 b are thick, the mechanical strength ofthe optical filter 230 is reinforced. However, if the oxidation films220 a and 220 b are excessively thick, the optical characteristics ofthe multi-layer thin film 230 are deteriorated. Therefore, the thicknessof the oxidation films 220 a and 220 b is preferably 1 to 2 μm.

The multi-layer thin film 230 can be stacked using dielectric mediums,such as SiO₂, ZrO₂, and TiO₂, or other materials. The multi-layer thinfilm 230 is deposited on the front surface of the oxidation film 220 a,and covers the recess 211 of the body 210.

The multi-layer thin film 230 can be made of a demultiplexing filterwhich divides a beam of light to series of beams with differentwavelengths. In addition, the thin film 230 can be made of amultiplexing filter which combines series of lights with differentwavelengths to one beam of light. Furthermore, the multi-layer thin film230 can be deposited with other thin films such as a band-pass filter,which passes only light of a predetermined wave, or a deflected lightbeam division filter.

The recess 211 is etched to have a depth from one side surface of thebody 210 such that the front and rear surfaces of the body 210 areexposed. In addition, the recess 211 is etched so that it makes contactwith the oxidation film 220 a of the front surface of the body 210.

FIGS. 3 a to 3 c are views explaining the optical filter shown in FIG. 2according to manufacturing steps. The optical filter 200 shown in FIG. 2is made of a wafer of silicon material shown in FIG. 3 a. It ispreferable to use a wafer 310 that is polished on both sides. Thethickness of the optical filter 200 is determined according to thethickness of the wafer, and the thickness of the wafer 310 can beselected as occasion demands.

Referring to FIG. 3 a, oxidation films 320 a and 320 b, which areoxidized at high temperature, are formed on the first and secondsurfaces of the wafer 310, and a multi-layer thin film 340 is depositedon the oxidation film 320 a formed on the first surface of the wafer310.

FIG. 3 b shows a status that a plurality of sectors 330 which makes upthe wafer 310 are divided into wafer pieces 311 by dicing or othermethods.

FIG. 3 c shows a status that a trench 313 is formed by etching a part ofthe body of each the wafer piece 311 and the second oxidation film 320b. The wafer piece 311 is finished by cutting a center of the trench 313according to the cutting line of A and B shown in FIC 3 c. The trench313 can be formed by Bosch process which is mainly used for deep etchingsilicon.

FIG. 4 is a view showing an optical filter according to the secondembodiment of the present invention. Referring to FIG. 4, the opticalfilter 400 includes a body 410, a multi-layer thin film 422 situated atthe front side of the body 410, and a recess 411 formed in the body 410.

The recess 411 is formed by anisotropic etching. The area of the recess411 in contact with the oxidation film 421 and the area of the recess411 exposed to the body are different.

The optical filter 400 is manufactured by using a wafer with oxidationfilms on both surfaces. It is manufactured by depositing the multi-layerthin film 422 on the oxidation film 421, by forming a trench of apredetermined shape, and by cutting the wafer into a plurality ofoptical filters. The multi-layer thin film 421 deposited on oxidationfilms includes a plurality of dielectric medium. Moreover, the trench ofa predetermined shape may be formed by etching the wafer.

FIG. 5 shows a step where patterns for cutting the wafer into opticalfilters are formed. The method of manufacturing the optical filter shownin FIG. 5 is similar to the method shown in FIGS. 3 a and 3 b. Onlydifference between two methods is that in FIG. 5, the trench 511 isformed by and cut simultaneously. Specifically, in FIG. 5, the trench513 is formed in the wafer piece 511 by etching and the wafer piece 511that contains the trench 513 is cut into the optical filters 200 and400, as shown in FIGS. 2 and 4, at the same time.

The optical filter cut by etching generates less foreign substances thanthat cut by dicing or other mechanical cutting methods. Moreover,etching introduces less physical force on the multi-layer thin film.Etching, therefore, can be used to manufacture an optical filter withwide area.

FIGS. 6 to 8 are views showing an optical filter according to the thirdembodiment of the present invention. Particularly, they show an opticalfilter as it engages a planar lightwave circuit. Referring to FIGS. 6 to8, the planar lightwave circuit 620 comprises first and secondwaveguides 622 a and 622 b that are separated by a predetermineddistance, and the optical filter positioned between the first and thesecond waveguides. The first and second waveguides comprises an upperclad, an active layer, and a lower clad that are sequentially stacked ona semiconductor substrate 621.

The optical filter 610 comprises a body 611, a recess 611 a, amulti-layer thin film 613, and oxidation films 612 a and 612 b. The body611 is of a polyhedron structure containing a recess 611 a withpredetermined depth from one side surface of the body. The front andrear surfaces of the recess is exposed. The multi-layer thin film 613 isdeposited on the front surface of the body 611 and is formed on thefront surface of the exposed recess 611 a of the body 611. Oxidationfilms 612 a and 612 b are formed on both surfaces of the body. One ofthe oxidation films 612 a and 612 b is formed between the multi-layer613 and the body 611.

The multi-layer thin film 613 is positioned between the first and secondwaveguides 622 a and 622 b as the second waveguide 622 b is insertedinto the recess 611 a. A portion of one surface of the body 611, onwhich the recess 611 a is exposed, makes contact with the semiconductorsubstrate 621.

Compared to an optical filter in which a multi-layer thin film isdeposited on a thin substrate, the present invention offers severaladvantages. First, the optical filter according to the present inventionreduces the manufacturing cost. Particularly, the present inventionreduces the deposition cost by using a wafer made of a silicon materialand by manufacturing a plurality of optical filters simultaneously.Second, the body under current invention supports the multilayer thinfilms. Such support renders deep etching of the recess, a process neededto enable the recess to engage with the planar circuit, unnecessary.

The first and second waveguides 622 a and 622 b are linear type fromwhich a portion is removed, but can be grown to a planar type coveringthe upper side of the semiconductor substrate 621. If the first andsecond waveguides 622 a and 622 b are grown to the planar type, agroove, in which the body 611 is inserted, is formed on the first andsecond waveguides 622 a and 622 b.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. An optical filter comprising: a body of a polyhedron structure inwhich a recess having a predetermined depth from one side surface isexposed on a front surface and a rear surface; and a multi-layer thinfilm which is deposited on the front surface of the body to cover anexposed portion of the recess of the body.
 2. The optical filteraccording to claim 1, further comprising oxidation films formed betweenthe front surface of the body and the multi-layer thin film and formedon the rear surface of the body.
 3. The optical filter according toclaim 1, wherein the body is made of a silicon material.
 4. The opticalfilter according to claim 1, wherein a plurality of dielectric mediumsare stacked in the multi-layer thin film.
 5. The optical filteraccording to claim 1, wherein a metallic medium is stacked in themulti-layer thin film.
 6. The optical filter according to claim 1,wherein the multi-layer thin film is a multiplexing filter.
 7. Theoptical filter according to claim 1, wherein the multi-layer thin filmis a demultiplexing filter.
 8. The optical filter according to claim 1,wherein the multi-layer thin film is a band-pass filter.
 9. The opticalfilter according to claim 1, wherein the multi-layer thin film is adeflected light beam division filter.
 10. The optical filter accordingto claim 2, wherein the thickness of the oxidation films are 1 to 2 μm.11. A method for manufacturing an optical filter, the method comprisingthe steps of: forming oxidation films by oxidizing first and secondsurfaces of a wafer; depositing a multi-layer thin film on the oxidationfilm formed on the first surface; forming a trench by etching the waferso as to make contact with the oxidation film of the first surface and acentral portion of the second surface; and cutting the first surface,the second surface, and the trench to form more than two optical filterseach of which has a body.
 12. The method according to claim 11, whereinthe step for forming the oxidation films on the surfaces of the waferfurther comprises the step for forming the film on a silicon wafer. 13.The method according to claim 11, wherein the step for forming theoxidation films on the surfaces of the wafer further comprises the stepof forming the film on the wafer that is polished on both sides.
 14. Themethod according to claim 11, wherein the step of cutting the wafer intooptical filters each of which has a body comprises providing a pathformed on the multi-layer thin film and dicing according to the path.15. The method according to claim 11, wherein the step for forming thetrench comprises wet etching.
 16. The method according to claim 11,wherein the step for cutting the wafer into more than two opticalfilters comprises bulk etching.
 17. The method according to claim 12,wherein the step for forming the trench and the step for cutting thewafer occur simultaneously.
 18. A planar light wave circuit whichcomprises first and second waveguides separated by a predetermineddistance on a semiconductor substrate, wherein the planar light wavecircuit further comprises a body of a polyhedron structure in which arecess having a predetermined depth from one side surface is exposed ona front surface and a rear surface, and a multi-layer thin film which isdeposited on the front surface of the body to covers an exposed portionof the recess of the body, and the multi-layer thin film is positionedon the semiconductor substrate so as to be opposed to first and secondwaveguides by inserting the second wave guide into the recess.
 19. Theplanar light wave circuit according to claim 18, further comprising anoxidation film formed between the body and the multi-layer thin film.20. The planar light wave circuit according to claim 18, wherein aportion of the surface of the body on which the recess is exposed andmakes a contact with the semiconductor substrate.